Field of the Invention
The present invention relates generally to the field of orthodontics. More specifically, the present invention discloses a tooth-positioning appliance for closing a space between teeth.
Statement of the Problem
A wide variety of orthodontic aligners have been used for many years in repositioning teeth during orthodontic treatment. It should be noted that the terms “aligner”, “positioner” and “tooth-positioning appliance” are largely synonymous as used in the orthodontic field.
This type of orthodontic treatment typically involves separate tooth-positioning appliances for the upper and lower teeth. The tooth-positioning appliances fit over the teeth, covering nearly all of the facial and lingual surfaces, and also most of the occlusal, or biting surfaces of the teeth. The early positioners described in the prior art were made from a set of plaster models derived from three-dimensional negative dental impressions of the patient's teeth. The plaster dental models were modified by cutting the teeth apart using a small jeweler's saw or rotary cutting discs and repositioning the plaster teeth in a better, straighter, desired arrangement, and holding the teeth in the new arrangement by using dental wax. The reset teeth molds provide the basis for manufacturing the positioners. The resilience of the material from which the positioner is made provides the energy to move the teeth from their original position toward the new straightened position. From the earliest disclosure of the tooth positioner, many of the proposed designs in the prior art have shown moving the teeth in a series of incremental steps. Making a series of appliances is difficult if the tooth arrangement for each step must be made by hand using plaster and wax.
Starting in the early 1990's, digital technologies have begun to provide orthodontists with fundamentally new tools for delivering orthodontic treatment by fabricating tooth models in small but accurate incremental steps. Commercially-available CAD/CAM (computer-aided design and manufacturing) software can produce the desired tooth models, from which a progressive series of appliances can be manufactured. These tools include 3D imaging of the patient's dentition, and CAD/CAM systems for creating virtual models in orthodontic treatment to then produce customized orthodontic appliances.
An example of the successful orthodontic application of these digital technologies is seen in the commercial service known as the Invisalign® program by Align Technology, Inc. of San Jose, Calif. The Invisalign program is largely based on U.S. Pat. No. 5,975,893 (Chishti et al.) and many related patents, including U.S. Pat. No. 6,398,548 (Muhammad et al.). Invisalign tooth positioners are a progressive series of thin, transparent, U-shaped plastic appliances formed over computer-generated forming patterns grown from a virtual model of the patient's dental anatomy. The process for forming aligners uses a combination of vacuum, pressure and heat. This forming process is informally referred to within the orthodontic laboratory community as the “suck down” process.
In order to produce a series of Invisalign-type tooth aligners, a technician first scans a patient's upper and lower model set to obtain CAD-manipulatable virtual models of a patient's dental anatomy. Alternative methods for obtaining a 3-D virtual image of a patient's dental anatomy include: (1) using an industrial C-T machine to directly scan a negative 3-D impression made from a stable silicone rubber or polyvinyl siloxane material without pouring up a plaster model (the method currently favored by Invisalign) or (2) directly light-scanning a negative impression made from any material of the patient's teeth; or (3) directly scanning the patient's teeth with an intra-oral scanner. All of these methods are currently available and widely used. A model set normally consists of one upper and one lower plaster model of the teeth, palate and gums. Once the virtual model of the original malocclusion has been obtained, a technician will then undertake steps involving extensive manipulation of the virtual malocclusion. This involves extensive repositioning of the teeth according to a comprehensive and sequential procedure, ultimately arriving at a finished or ideal occlusion for that patient. The finished occlusion in the virtual model is consistent with the complete repositioning of the patient's upper and lower occlusion that would result at the end of successful conventional orthodontic treatment.
After the steps described above are accomplished, the technician possesses two versions of the patient's teeth available within the virtual CAD environment. One version represents the original malocclusion and the other represents the ideal occlusion. In other words, the technician has the beginning and the end states.
The next step in the Invisalign process involves the creation of an incremental, progressive series of physical forming models. Each of these forming models represents a snapshot of the patient's future occlusion at specific incremental steps along the patient's proposed treatment sequence between the beginning and the end conditions as described above. To accomplish this, the technician creates a virtual “first transition model” that sees a slight repositioning of all or most of the teeth. This first transition model sees some or all of the teeth being subtly moved from their original pre-treatment positions to a virtual first transition position that is in the direction of their intended finished positions. Similarly, a second virtual transition model is created that sees the virtual teeth being moved again slightly further in the desired directions. The objective of the Invisalign technician is to create a series of progressive models, each biased slightly further than the previous one, and each moving the teeth slightly closer to their finished target positions. A final forming model will take the teeth from the series of transition positions and move them into their final, desired positions.
Once such a series of virtual intermediate forming models has been created and a final forming model has been created by the Invisalign technician, the digital code representing each of the models in the series is directed to operate a computer numerically-controlled (CNC) machine known as a rapid prototyping machine. Within a rapid prototyping machine, the series of physical forming models are grown using any of number of conventional processes, such as stereo lithography or 3D printing. The growing step results in the production of hard, physical duplicates of each of the series of virtual intermediate models and the final model.
The next step of the Invisalign process sees each of the series of physical models being in turn mounted in a suck-down machine where a combination of pressure, heat and vacuum is used to form the actual series of progressive aligners from plastic sheet material of a constant thickness. Once the series of progressive aligners are formed and trimmed, they are sequentially labeled, packaged and shipped to the attending orthodontist. The orthodontist then schedules an appointment for the patient, at which time the aligners and instructions for their use are given to the patient. The patient is instructed to wear the first set of aligners for a period of time, typically two weeks. After that, the first set is discarded and the patient transitions to the next set of the series and so on.
The aligners serve to urge the patient's teeth to move according to the positional biases created virtually by the Invisalign technician. The teeth are progressively biased and urged to move in desired directions toward their predetermined finished positions by the resilience of the polymeric material of the aligner. In response to the forces delivered by the aligners, certain physiological processes involving the creation and resorbtion of the bone supporting the roots of the teeth are initiated. The net result is the slow, progressive orthodontic movement of the roots of the teeth through the underlying bone toward desirable positions and orientations.
Progressive thin-shell aligners have proven to be very effective in treating some types of orthodontic cases, but they have shortcomings in other situations. In particular, a significant percentage of orthodontic treatments require tooth extractions for various reasons, such as severe crowding of the patient's teeth. Percentages are reported to range from 25% to 50% in some practices.
When severe dental crowding is present, the teeth most often removed to create space are the first premolars. Orthodontists do not like removing anterior teeth (cuspids or incisors) primarily because they are highly visible, but even after all extraction spaces are closed, it is often obvious if an anterior tooth is missing unless the teeth can be reshaped. The front teeth have a distinctive appearance and other teeth substituted in their place often don't quite fit there. On the other hand, removal of large molar teeth presents difficulties with space closure and usually the crowding is in the front part of the mouth. Removing two or four premolars (the teeth in the middle of each side) allows a satisfactory appearance and is more easily accomplished. Occasionally, for various reasons, orthodontists may still find it necessary to remove either anterior teeth or molars, and occasionally these teeth are already missing prior to orthodontic treatment.
Closing significant spaces in orthodontics presents a challenge, whether the spaces were produced by extracting teeth as part of the orthodontic treatment plan, or whether the spaces were present prior to treatment.
Orthodontists usually want to completely close extraction sites by moving the teeth on either side of the extraction site together, and it is desired to have parallel roots on all of the teeth after the extraction sites have been closed. With conventional aligners, as the teeth adjacent to the extraction site are moved, the crowns of the teeth tend to tip toward each other, and the roots, where the greatest resistance to movement is encountered, tend to trail behind. The problem is caused primarily by poor engagement of the aligner on teeth that are adjacent to tooth extraction sites. The other teeth, farther away from the extraction sites also tend to tip as well, again because the aligner does not engage the teeth very well. In other words, the roots fail to move as much as the crowns of the teeth move. Therefore, a need exists for a removable orthodontic appliance that can be used during the space-closure stage of treatment without tipping adjacent teeth into the extraction sites.
Solution to the Problem. The present invention addresses this shortcoming in the prior art by providing a removable appliance for use in the space-closure stage of orthodontic treatment. It is capable of closing spaces without tipping teeth with a similar degree of control to that achieved with fixed braces. In particular, the present appliance is divided into anterior and posterior appliance segments on groups of teeth on either side of the extraction sites. Each appliance segment removably engages a selected set of teeth aided by attachments bonded to the teeth. Elongated, thin elastomeric tabs extend laterally from selected appliance segments to span the extraction sites and slide in corresponding flat receptacles (e.g., tubes or molded plastic receptacles) specially shaped to receive the tabs on other appliance segments. These elastomeric tabs allow relative anterior-posterior movement of the appliance segments and their associated groups of teeth to close the extraction sites. The flat shape of the tabs and slots, and the strength of the appliance segments result in an assembly that is fairly rigid in a vertical plane. This provides improved leverage to keep teeth in an upright position and to prevent the tipping of adjacent teeth into the extraction sites.